National Instruments Image Acquisition Software - Manual (Page 27)

Chapter 3 Programming with NI-IMAQ
© National Instruments Corporation 3-7 NI-IMAQ User Manual
High-Level Snap Functions
A snap acquires a single image into a memory buffer. Snap functions
include
imgSnap and imgSnapArea . Use these functions to acquire a
single frame or field to a buffer. To use these functions, you must have a
valid session handle.
When you invoke a snap, it initializes the board and acquires the next
incoming video frame (or field) to a buffer. A snap is appropriate for
low-speed or single-capture applications where ease of programming
is essential. Figure 3-2 illustrates a typical snap programming order.
Figure 3-2. Snap Programming Flowchart
The hlsnap.c example demonstrates how to perform a single snap using
imgSnap . The example opens an interface and a session and then performs
a single snap. The buffer pointer that is passed to imgSnap is initialized to
NULL, which instructs imgSnap to automatically allocate a buffer for the
image. The size of the buffer is calculated based on the region of interest
(ROI) and the rowPixel attributes: ROI height multiplied by rowPixel
multiplied by the number of bytes per pixel. When you open a session, the
ROI values are initialized from the acquisition window (ACQWINDOW)
dimensions that are configured in the IMAQ Configuration Utility. The
ACQWINDOW dimensions will vary depending on the type of camera you
are using.
imgInterfaceOpen
imgInterfaceOpen opens
and configures the interface
according to the file setup by the
IMAQ Configuration Utility.
imgSessionOpen opens a session that
will be used for all acquisition and configuration.
User-specific image processing
imgClose closes the interface and session.
imgSnap acquires an image into a buffer. The
buffer may be passed into imgSnap or automatically
allocated by NI-IMAQ. If no buffer is passed into
imgSnap , it will automatically snap into a new buffer
and return a pointer to the buffer.
imgSessionOpen
imgSnap
User-Specific Functions
imgClose
UM.book Page 7 Monday, July 13, 1998 9:49 AM